Stop using non-existent method of Mock
Stop using non-existent method of Mock

There is no method called_once_with() in Mock object.
Use assert_called_once_with() instead.

I tested it in my tox environment. The revised assertion will work.

Change-Id: I530b58a434b4c031d236b0b04e8c7091854c992f
Closes-Bug: #1544522

openstack/python-designateclient